Online monitoring, indicating and controlling flammable, explosive, toxic and hazardous gases have been one of the important issues to be solved urgently for the high risk industries, such as coal mining, petroleum mining, chemistry and thermal power generation. In addition, a system to monitor and control toxic and hazardous gases is expected for maintaining a clean environment, with the improvement of living standards of human being. As a result, the development of a system for sensing and monitoring hazardous gases is essential and becomes a key topic in the field of sensing technology.
Gas detection method working with optical fiber sensing technique, especially near infrared absorption spectrum quantitative detection, has developed rapidly in the last two decades. As a novel optical fiber gas detection method provided in this invention, optical fiber active intra-cavity method is adopted. In this method, a gas cell is placed within a resonant cavity of an optical laser, and the wavelength of the laser varies with the absorption spectrum of the detected gas. During the process of laser generation by reciprocating resonance of weak optical signals inside the resonant cavity, the weak optical signals pass through the gas frequently, which leads to a fairly long effective optical path in the small gas cell. By this way, gas detection sensitivity is significantly improved. The system continuously scans the entire gain band of the laser. All the absorption spectrums of the gases inside the chamber are thus obtained. Hence, the system is capable of sensing different gases simultaneously.
In general, optical fiber containing rare earth is used as gain medium of an optical fiber laser. At present, the commonly used rare earth ions comprise Nd3+, Ho3+, Er3+, Tm3+, Yb3+ and so on. Optical fiber doped with Nd3+ has maser band at wavelength of 1.088 μm, optical fiber doped with Ho3+ has maser band at wavelength of 2.1 μm, optical fiber doped with Er3+ has high gain at wavelength of 1.55 μm, optical fiber doped with Tm3+ has maser band at wavelength of 1.4 μm, and, optical fiber doped with Yb3+ has relative wide excitation band at wavelength range of 0.97 μm-1.2 μm. The above statements indicate that the excitation band of optical fiber doped with rare earth covers almost entire near infrared bandwidth. However, the gain band of one optical fiber doped with rare earth elements covers only part of the bandwidth, with limited applicability in terms of gas types. To detect more gases, individual and separate optical fiber laser intra-cavity gas sensing systems with different bands should be applied.
This invention aims to provide a system capable of detecting more types of gases simultaneously, by expanding the scanning range of wavelength by combining the gain bands of a number of optical fibers doped with different rare earth materials.